Eric Harroun, 30, of Phoenix was arrested Tuesday night by the FBI at a hotel near Washington Dulles International Airport in Virginia.

Courtesy ABC15

WASHINGTON -- A former U.S. soldier has been arrested and charged with illegally using a weapon on behalf of an al Qaeda-affiliated group in Syria.

Eric Harroun, 30, of Phoenix was arrested Tuesday night by the FBI at a hotel near Washington Dulles International Airport in Virginia. A Justice Department official tells CNN that FBI agents questioned Harroun at the hotel, then took him into custody.

Harroun appeared Thursday in federal court in Alexandria, Virginia, and was charged in connection with his alleged use of a rocket-propelled grenade in Syria.

The law used to charge him states, "Any national of the United States who, without lawful authority, uses or threatens, attempts, or conspires to use a weapon of mass destruction outside of the U.S. shall be imprisoned for any term of years or for life, or if death results, may be punished by death."

Harroun served with the U.S. Army from 2000 to 2003. He is not charged with targeting U.S. troops in Iraq.

The organization he allegedly fought with, al-Nusra Front, is one of several aliases used by the al Qaeda in Iraq terrorist organization. The organization claims responsibility for nearly 600 terrorist attacks in Syria, the Justice Department said.

An FBI affidavit says Harroun crossed into Syria in January 2013 and fought against Syrian President Bashar al-Assad's forces. He posted photos and videos of himself on the Internet handling RPGs and other weapons, it said.

The Pentagon declined to comment on Harroun's arrest. However, "It's always a concern when terrorist networks in that part of the world and elsewhere seek to recruit Americans, whether they're in the military or not," spokesman George Little told CNN's Erin Burnett.

"I don't think this is a widespread phenomenon, and most of our people in this country -- and certainly most men and women in the military -- would not consider joining a terrorist network," Little added.